1. Field of Invention
This is a further development in the art of thermionic ionization detectors, and provides a method and apparatus for detecting specific chemical substances in a gaseous environment by ionizing these substances on the surface of a heated, multiple-layered thermionic source.
2. Prior Art
Thermionic ionization detectors are used in the field of gas chromatography and elsewhere for the detection of specific chemical substances in a flowing gas stream. Such detectors usually consist of the following components: a thermionic source comprised of a surface impregnated with an alkali metal compound and heated electrically by means of a fine metallic heating wire embedded in the source; an electronic power supply capable of supplying an electrical heating current to the source; a collector electrode structure adjacent to but separated from the source; a gas stream flowing past the thermionic source; a means of electrically polarizing the source to cause either positive or negative ions formed on the surface of the source to migrate through the gas stream to the collector electrode; and an electronic current-measuring circuit such as an electrometer to measure the current arriving at the collector electrode. The single most important component in this detector is the thermionic source, and much of the prior art in thermionic detection techniques has dealt with methods of improving the construction and performance of the thermionic sources.
In 1951, Rice (U.S. Pat. No. 2,550,498) described a method and apparatus for electrically detecting vapors of certain substances by sensitizing a hot surface with a material from the class of alkali metals and their compounds. In Rice's apparatus, the heated sensitized surface consisted of a metallic heater coil wound on an alumina ceramic cylinder. Natural alkali impurities within the alumina ceramic served to produce the required sensitizing action for short operating times. Rice taught that the active life of the sensitized alumina could be increased or restored by soaking the alumina in a water solution containing an alkali metal salt. For even longer life, Rice further taught that the alumina cylinder could be replaced by a cylinder of alkali glass composition such as that described by Blewett (Physical Review, Vol. 50, p. 464, 1936).
In 1957, Roberts (U.S. Pat. No. 2,795,716) described an improved detector featuring a sensitized source having longer life compared to that described by Rice. Roberts' source consisted of a cylindrical alumina ceramic core upon which was wound a heater coil. The alumina core and heater coil were coated on their outer surfaces by a layer of "positive ion emitting material". This coating material was formed from an alkali glass which was powdered and mixed with a suitable ceramic cement in a desired proportion, then coated over the heating coil and alumina core and allowed to set.
In 1975, Kolb and Bischoff (U.S. Pat. No. 3,852,037) described a selective ionization detector which used an alkali glass material deposited in the form of a bead onto an electrical heating wire. Kolb and Bischoff argued that successful detection required operating the alkali glass in a heated, softened state such that molecular motion within the body of the bead acted to maintain an adequate supply of alkali material at the bead surface. Kolb and Bischoff collected negative ionization whereas the earlier devices of Rice and Roberts collected positive ionization.
In 1977, Burgett et. al. (Journal of Chromatography, Vol. 134, p. 57, 1977), described a nitrogen-phosphorus specific detector which used an electrically heated source comprised of a ceramic cylinder core coated with a surface layer of an alkali salt activator similar to the alkali-glass described earlier by Rice. In Burgett's source, a segment of the heating coil was embedded in the ceramic core, and positive ions were collected.
In 1978, Patterson (Journal of Chromatography, Vol. 167, p. 381, 1978) presented data demonstrating that the ionization mechanism in these thermionic detectors was a surface ionization process rather than a gas phase process. According to Patterson, sample compounds or their decomposition products extract electrical charge from the hot thermionic surface, and the resulting ionization is collected at an adjacent electrode. For such a surface ionization process, the three most important operating parameters in the detector were identified to be the work function of the thermionic surface, the temperature of the surface, and the chemical composition of the gas environment surrounding the surface.
In 1980, Patterson (U.S. Pat. No. 4,203,726) described a thermionic detector in which the source was formed from a homogeneous mixture of an alkali metal compound and a ceramic cement coated directly over a helical shaped heating coil. In this case, the alkali-ceramic material formed the entire body of the source rather than just the surface layer. Similar to Kolb and Bischoff, it was argued that the presence of alkali material within the body of the source helped promote longer source operating life by providing a reservoir for replenishing the alkali concentration at the source's surface.
In 1982, Patterson (Journal of Chromatographic Science, Vol. 20, p. 97, 1982) described a thermionic source which contained a separate metallic heating wire and a separate metallic temperature sensing wire. The two separate wires were contained in a four-hole alumina ceramic cylindrical core, and the surface of the core was coated with an activating alkali-ceramic mixture. The advantage of this source construction was that it provided a means of controlling the source temperature with a constant temperature electronic circuit rather than a constant circuit.
Since the mechanism of ionization in the thermionic detector is a surface ionization process, most of the prior art has concentrated on the development of suitable surface compositions in order to obtain the specific responses of the detector. However, in addition to the surface ionization, there must also occur a process of charge migration in the source in order to replace the electrical charge lost from the surface. This charge migration can be described as a current originating at the metallic heating wire and flowing through the body of the source to its surface. For the prior art devices in which the source was composed of a homogeneous composition of an alkali-glass or alkali-ceramic formulation, the presence of alkali material in the source body facilitated the flow of current through the source body. However, such alkali-glass or alkali-ceramic compositions were chosen primarily for their surface ionization characteristics and did not necessarily provide the most optimum medium for the process of charge migration through the source body. A particular disadvantage of these homogeneous source compositions was that the hot metallic heating element was exposed to corrosive attack by direct contact with alkali atoms.
This corrosion problem was minimized by those prior art devices in which the heating element was embodied in an inert alumina ceramic core and the alkali-glass or alkali-ceramic sensitizing material was present in the form of a surface layer on the core. However, in these devices, the core material was not conducive to the conduction of current through the source body. Consequently, a newly constructed source with an inert alumina ceramic core, generally had to be conditioned at operating temperatures for a time of approximately 24 hours or more before the desired detector responses were obtained. During this conditioning period, it could be postulated that there occured some permeation of alkali material from the source surface layer into the source core, thereby enhancing the electrical conductivity of the core until some equilibrium condition was reached. Therefore, charge migration through the body of the resultant conditioned source was dependent on the composition of the alkali-impregnated surface layer as well as on the operating conditions used during the conditioning period.
In the 1982 prior art device described by Patterson, the metallic heating wire was contained inside two of four tubular holes in an alumina ceramic cylindrical core. Since these tubular holes had to be of larger internal diameter than the diameter of the heating wire in order to allow the heating wire to be guided through the alumina core, a continuous physical contact of the heating wire and the alumina core could not be ensured. This undefined extent of physical contact presented a further negative variance affecting the migration of charge from the heating wire, through the alumina core to the alkali-impregnated surface layer.
In some of the prior art devices, the thermionic sources were constructed in such a manner that portions of the metallic heating wires were not coated with the alkali-glass or alkali-ceramic material, nor with any insulating material. Since the metallic heating wires were typically wires of very small diameter, such exposed fine wires were often subject to mechanical breakage during installation or operation of the source. Also, such exposed wires were subject to corrosion from various chemicals present in the gas stream being measured.